1. Field of the Invention
The present invention relates to an image reading apparatus for reading an image formed on a sheet, and in particular to an image reading apparatus which is capable of reading an image up to a trailing edge of a sheet in an image reading position of the sheet, and an image forming apparatus provided with the same.
2. Related Background Art
Conventionally, as shown in FIG. 8, examples of an image reading apparatus include one adapted to convey a sheet having an image formed thereon (hereinafter referred to as “original”) D with a pre-reading roller 132, a post-reading roller 142, and the like, which are arranged along a conveying direction of the original D, and to read the image on the original D with an original reading portion 104 in an image reading position 127a in the course of conveyance of the original D (e.g., see Japanese Patent Application Laid-Open No. 2000-038234). Then, this image reading apparatus 101 is adapted to read the original D while pressing the original D against the image reading position 127a with a biasing member 147 such that the original D does not lift in the image reading position 127a. 
However, the conventional image reading apparatus disclosed in Japanese Patent Application Laid-Open No. 2000-038234 has the following problems which should be solved.
The pre-reading roller 132 and the post-reading roller 142 are provided as rollers for conveying the original D on the upstream side and the downstream side of the original conveying direction of the image reading position 127a. An original conveying form of the pre-reading roller 132 and the post-reading roller 142 in such an original conveying mechanism is as described below.
That is, the original D is conveyed only by the pre-reading roller 132 until a leading edge of the original D reaches the post-reading roller 142. Then, when the leading edge of the original D has reached the post-reading roller 142, the original D is conveyed by the pre-reading roller 132 and the post-reading roller 142. Finally, after a trailing edge of the original D comes out of the pre-reading roller 132, the original D is conveyed by the post-reading roller 142 until the original D is discharged. Note that in the series of operations, for simplification of explanation, it is assumed that a separation roller 138, which separates the originals D one by one and conveying the originals D to the pre-reading roller 132, does not contribute to conveyance of the originals D.
In addition, a main conveyance roller for mainly conveying the original D is often one of the pre-reading roller 132 and the post-reading roller 142. Usually, the pre-reading roller 132 is adapted to function as the main conveyance roller.
In this case, original conveying forces of the pre-reading roller 132 and the post-reading roller 142 are in a relation in which the original conveying force of the pre-reading roller 132 is larger than the original conveying force of the post-reading roller 142.
Note that the original conveying force of the pre-reading roller 132 depends upon a force of nipping the original D, friction with the original D, and the like of the pre-reading roller 132 and a roller 132a. In addition, the original conveying force of the post-reading roller 142 depends upon a force of nipping the original D, friction with the original D, and the like of the post-reading roller 142 and a roller 142a. 
In addition, a slight fluctuation in a dimension (usually, approximately ±0.04 mm, and, even if importance is placed on accuracy, approximately ±0.02 mm) may occur in an external dimension, which affects a conveying amount of an original, when the pre-reading roller 132 and the post-reading roller 142 are manufactured.
Therefore, it is difficult to make conveying amounts of the original of the pre-reading roller 132 and the post-reading roller 142 identical to each other.
Thus, the conveying amount of the original of the pre-reading roller 132 and the post-reading roller 142 are in a relation in which the conveying amount of the original of the pre-reading roller 132 is equal to or smaller than the conveying amount of the original of the post-reading roller 142.
According to such a relation, after the leading edge of the original has reached the post-reading roller 142, the image reading apparatus 101 conveys the original D with the pre-reading roller 132 and the post-reading roller 142 while sliding the post-reading roller 142 against the original D, and always pulling the original D to bring the original D into close contact with reading means.
Consequently, when the trailing edge of the original D comes out of the pre-reading roller 132 after the pre-reading roller 132 and the post-reading roller 142 has rotated several times, conveying load of the post-reading roller 142 slidingly conveying the original D is released all at once. In an instance when the conveying load is released, the post-reading roller 142 may convey the original D with a conveying amount exceeding a predetermining conveying amount.
As described above, it is likely that the conventional image reading apparatus 101 causes unevenness of conveyance among the originals D.
As a result, in the image reading apparatus 101, original conveying accuracy declines markedly, an image in original reading is deteriorated markedly, and original reading accuracy also declines.
In particular, with a scanner function, an image read once can be enlarged by several times to be shown on a monitor by a personal computer (PC). Therefore, the read image can be identified in a unit of one pixel. Thus, degradation of quality of the read image may be seen conspicuously.
As a measure to solve such a problem, it is possible to end reading of the original D while the original D is conveyed by the pre-reading roller 132 and the post-reading roller 142 before the original D comes out of the pre-reading roller 132. However, in this case, another problem occurs in that the image cannot be read surely up to the trailing edge of the original D.